powershell-windows – Other skill for AI agents

PowerShell Windows Patterns

> Critical patterns and pitfalls for Windows PowerShell.

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1. Operator Syntax Rules

CRITICAL: Parentheses Required

| ❌ Wrong | ✅ Correct |

|----------|-----------|

| if (Test-Path "a" -or Test-Path "b") | if ((Test-Path "a") -or (Test-Path "b")) |

| if (Get-Item $x -and $y -eq 5) | if ((Get-Item $x) -and ($y -eq 5)) |

Rule: Each cmdlet call MUST be in parentheses when using logical operators.

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2. Encoding Compatibility

Prefer portable log markers

| Purpose | ❌ Don't Use | ✅ Use |

|---------|-------------|--------|

| Success | ✅ ✓ | [OK] [+] |

| Error | ❌ ✗ 🔴 | [!] [X] |

| Warning | ⚠️ 🟡 | [*] [WARN] |

| Info | ℹ️ 🔵 | [i] [INFO] |

| Progress | ⏳ | [...] |

PowerShell supports Unicode. ASCII log markers are a portability choice, not a syntax requirement. For non-ASCII scripts read by Windows PowerShell 5.1, use UTF-8 with BOM; PowerShell 7 uses UTF-8 without BOM by default. Check the consuming tool before choosing file encoding.

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3. Null Check Patterns

Always Check Before Access

| ❌ Wrong | ✅ Correct |

|----------|-----------|

| $array.Count -gt 0 | ($null -ne $array) -and (@($array).Count -gt 0) |

| $text.Length | if ($null -ne $text) { $text.Length } |

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4. String Interpolation

Complex Expressions

"Value: $($obj.prop.sub)" is valid subexpression interpolation. Store a complex expression in a variable when that improves readability.

Pattern:

$value = $obj.prop.sub
Write-Output "Value: $value"

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5. Error Handling

ErrorActionPreference

| Value | Use |

|-------|-----|

| Stop | Required cmdlet failures must enter catch |

| Continue | Report a non-terminating error and continue deliberately |

| SilentlyContinue | When errors expected |

Try/Catch Pattern

• A return inside try is valid; finally still runs
• Use finally for cleanup
• Return after try/catch

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6. File Paths

Windows Path Rules

| Pattern | Use |

|---------|-----|

| Literal path | C:\Users\User\file.txt |

| Variable path | Join-Path $env:USERPROFILE "file.txt" |

| Relative | Join-Path $ScriptDir "data" |

Rule: Use Join-Path for cross-platform safety.

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7. Array Operations

Correct Patterns

| Operation | Syntax |

|-----------|--------|

| Empty array | $array = @() |

| Add item | $array += $item |

| ArrayList add | $list.Add($item) | Out-Null |

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8. JSON Operations

CRITICAL: Depth Parameter

| ❌ Wrong | ✅ Correct |

|----------|-----------|

| ConvertTo-Json | ConvertTo-Json -Depth 10 |

Rule: Always specify -Depth for nested objects.

File Operations

| Operation | Pattern |

|-----------|---------|

| Read | Get-Content "file.json" -Raw | ConvertFrom-Json |

| Write | $data | ConvertTo-Json -Depth 10 | Out-File "file.json" -Encoding UTF8 |

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9. Common Errors

| Error Message | Cause | Fix |

|---------------|-------|-----|

| "parameter 'or'" | Missing parentheses | Wrap cmdlets in () |

| "Unexpected token" | Parsing or quoting error | Inspect the reported token and file encoding |

| "Cannot find property" | Null object | Check null first |

| "Cannot convert" | Type mismatch | Validate input and convert to the required type |

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10. Script Template

# Strict mode
Set-StrictMode -Version Latest
$ErrorActionPreference = "Stop"

# Paths
$ScriptDir = Split-Path -Parent $MyInvocation.MyCommand.Path

# Main
try {
    # Cmdlet logic here; check $LASTEXITCODE after native executables.
    Write-Output "[OK] Done"
    exit 0
}
catch {
    Write-Warning "Error: $_"
    exit 1
}

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> Remember: Check shell version, encoding, null semantics and native exit codes. try/catch alone does not reliably turn native command failures into exceptions.

References: [encoding](https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_Character_Encoding), [quoting](https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_quoting_rules).

When to Use

Use when writing or debugging Windows PowerShell or PowerShell 7 scripts, especially command expressions, encoding, paths and error handling.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.